Camera tube having a photoconductive targetplate and luminescent surface on electrode for fixing dark current

ABSTRACT

A camera tube whose target plate is stabilized by slow electrons and whose dark current is comparatively low, is provided with a luminescent material which is arranged in the viewing field of the target plate at the area where the electrons returning from the target plate are captured so that the luminescent light produced by said electrons illuminate the target plate more or less uniformly.

United States Patent 1191 Weijland et a1. May 22, 1973 1 1 CAMERA TUBE HAVING A References Cited PHOTOCONDUCTIVE TARGETPLATE UNITED STATES PATENTS AND LUMINESCENT SURFACE ON ELECTRODE FOR FIXING DARK 2,564,737 8/1951 Szegho ..313/83 X CURRENT 2,673,305 3/1954 S'Zegh0.... 3l3/82 T X 2,927,234 3/1960 Kazan ..3l3/65 A [75] Inventors; Willem Paul Weijland; Gerardus 3,046,441 7/1962 De Vore ..3l3/65 R X Arnoldus Herman Maria vrijmn Jensen et al- A X both of Emmasingel Eindhoven 3,372,056 3/1968 De Haan et al.... .....3l3/65 A X Netherlands 3,445,826 5/1969 Myers ..3l3/65 A X 2,727,157 12/1955 Longinl 1 ..3l3/68 D X [73] Assignee: U.S. Philips Corporation, New 3,444,412 5/1969 De Haan et a1 ..313/65 A York, N.Y. Primary Examiner-Robert Segal [22] F1led. Nov. 16, 1970 Attorneyaprank R- Trifari [21] Appl. No.: 89,699

[57] ABSTRACT fl ign Application Priority Data A camera tube whose target plate is stabilized by slow Nov. 17 1969 Netherlands ..69l7279 electrons and whose dark current is comparatively low, is provided with a luminescent material which is 1521 us. c1. ..313/65 A, 313/92, 313/99 arranged in the viewing field of the target [511 Int. Cl "H013 31/38, Holj 29/18, H0 lj 31/40 area where the electrons returning from the target [58] Field of Search ..313/65 A, 65 R Plate are Captured the luminescent light produced by said electrons illuminate the target plate more or less uniformly.

4 Claims, 1 Drawing Figure Patented May 22, 1973 INVENTORS WE'JLAND GERARDUS A. HM. VRUSSEN WILLEM CAMERA TUBE HAVING A PHOTOCONDUCTIVE TARGETPLATE AND .LUMINESCENT SURFACE ON ELECTRODE FOR FIXING DARK CURRENT The invention relates to a camera tube comprising an electron gun and a photo-conductive target plate which is stabilized at cathode potential by an electron beam emanating from said gun, and which reflects noncaptured electrons of the electron beam in the form of a return beam.

If no special precautions are taken, the beam current in such camera tubes has to be adjusted at such a high value that a potential accumulation on the target plate due to the strong illumination of an image element is comparatively compensated for by a single scan. If the beam current is too low for this purpose, a signal pulse will appear during the next-following scan as a result of the residual charge, which pulse does not necessarily correspond to the scene at the instant concerned. Owing to a high beam current the potential at the area of parts only slightly exposed on the photo-conductive layer becomes, after a few scans, lower than the potential of exposed parts of the photo-conductive layer. This is due to the fact that a given percentage of electrons of the scanning beam emanate from the cathode with a velocity exceeding the average velocity by a few eV. These electrons, which form a tail at the top of the velocity distribution, are capable of attaining the photo-conductive layer, even if the latter has already assumed the cathode potential or a potential matching the average exit velocity and thus being lower. The potential of the target plate, which is also determined by the velocity distribution of the electrons in the scanning beam will lie in the range of the high-velocity electrons, when the dark current is low and hence the dark resistance is high in the photo-conductive layer. If an image element is slightly exposed after a time of nonexposure, its potential will slightly increase and at the scan of this image element a signal current corresponding to the extent of exposure had to flow. However, the resultant signal current does not satisfy this condition because despite the increase in potential of the image element concerned only high-velocity electrons of the tail of the distribution can impact upon the plate and their number is too low to annul the potential increase produced by the exposure. This gives rise to inertia in the camera tube.

The invention has for its object to eliminate these disadvantages by simple means and a camera tube in accordance with the invention is characterized in that at the area of impact of electrons of the return beam a luminescence material is arranged in the viewing field of the target plate, while the luminescent light produced by the electrons strikes the target plate at least partly.

This luminescent light produces an additional photocurrent in the photo-conductive layer so that even in regions not receiving light from the scene the potential is sufficiently raised for holding the working point for the beam within the range of the normal velocity distribution.

In a preferred embodiment a layer of luminescent material is applied to an electrode of the gun of the camera tube located in the viewing field of the target plate.

In a further preferred embodiment the luminescent material is applied to the side of a gauze facing the target plate in this camera tube.

It should be noted that in U.S. Pat. Specification No. 2,727,157 a camera tube of the Orthicon type is described, in which a layer of luminescent material is provided. The luminescent light serves, however, in this case for transferring the signal contents of electrons returned by a plate having image information as an optical signal across the glass wall of the tube to a receiver outside the tube. The layer of luminescent material is arranged so that the light emanating therefrom cannot attain the target olate.

Hereinafter one embodiment of the invention will be described with reference to the drawing, in which FIG. 1 is a schematic sectional view of one embodiment of a camera tube in accordance with the invention.

The camera tube of FIG. 1 is of the construction of the known Vidicon-type camera tubes. An elongated, cylindrical tube 1, having a glass envelope 2, encloses by an end face 3 having various through-connections 4, and a second end face 5 serving as the window through which the information-carrying light enters, an exhaust space 6. This space 6 comprises an electron gun 7 having a cathode 8, a control-grid 9 and a first anode 11, provided with a diaphragm 10. The tube comprises furthermore a cylindrical electrode 12, one end of which extends to near the first anode 11 and the other end of which is provided with a gauze 13 of electrically conductive material. The electron beam 14 produced by said electrode system scans a photo-conductive target plate 15 by means of electrostatic or electromagnetic focusing and deflection. The means for focusing and deflecting the beam 14 are not shown in the Figure.

The target plate 15 comprises a photo-conductive layer 16 of about 20 am in thickness, preferably made of PbO, and a signal electrode 17. The signal electrode may be adjusted to a potential exceeding the cathode potential by a few lOs of Volts and may be applied in the form of a transparent, electrically conductive layer directly to the window 5 of the camera tube. A signal 19 is derived from said signal electrode, for example, via a capacitor 18 and a resistor 20. A portion of the electrons of the scanning beam 13 is captured by the target plate, as is known, in accordance with the extent of exposure of the photo-conductive layer by the scene to be taken, whereas the remaining portion returns in the form of a return beam 21. A portion of this return beam strikes the gauze 13, whereas the electrons passing across the gauze are mainly captured by the anode 11. The return beam striking this electrode includes a residual deflection, as a result of which the beam scans a small domain around the diaphragm 10. Around the diaphragm 10 this electrode is provided with a layer of luminescent material 22. The luminescent light 23 produced by the electrons of the return beam in the layer 22 strikes the photo-conductive layer more or less uniformly and gives rise to an additional photo-current. The value of this additional photo-current depends upon the anode voltage, the current intensity of the return beam and the efficiency of the luminescent material, for example Cu-activated ZnS, or ZnSCdS. In choosing the luminescent material of the layer 22 also the wavelength sensitivity of the photo-conductive layer 16 has to be taken into account.

A portion of the luminescent light of the layer 16 will be captured in the embodiment described by the gauze 13. This loss of light may be avoided by applying the luminescent material to the side of the gauze 13 facing the target plate, while the gauze mesh has to be left open. For this purpose the gauze may be provided prior to mounting on the side concerned with luminescent material, for example by application from the vapour phase. The electrons captured by the gauze from the return beam give rise to luminescent light. The source of the luminescent light is nearer the target plate so that the loss of light is reduced. As a matter of course the two embodiments may be combined.

The embodiment in which the phosphor is applied to the gauze provides particular advantages when used in a tube in which during the fly-back time of the scanning beam the cathode is adjusted to a higher potential than during the scan itself. The electrons emanating from the cathode then have lower velocity and will be repelled for the major part by the target plate, whereas only the image elements highly charged owing to very strong exposure, the so-called high lights, will capture these electrons, so that the potential of these image elements is reduced and the disturbing effect of the high lights for the next scan is reduced. When during said fly-back time the electron beam is defocused, for example, by means of an additional electrode or by the high cathode potential itself, the electrons will strike a comparatively large portion of the target plate so that the resultant return beam will strike also a large portion of the gauze, luminescent light being thus produced over a comparatively large surface, which is conducive to the uniformity of the exposure of the photo-conductive layer.

What is claimed is:

l. A television camera tube comprising an evacuated envelope, a transparent conductive layer on the front wall of said envelope, an electron beam source and a target plate of photoconductive material on said conductive layer and positioned to be scanned by the electron beam within the envelope, said electron beam source having electron beam pervious electrode having an aperture therein for the passage of the electron beam and having a planar surface facing the target plate scanned by the electron beam and positioned to receive returning electrons from the target plate, and a luminescent material on said surface of said electrode facing, said target for receiving electrons returning from said target said electrons energizing said luminescent material to produce light which impinges on said target to maintain a predetermined dark current therethrough.

2. A camera tube as claimed in claim 1, in which the photo-conductive material of the target plate consists mainly of a layer of PhD of a thickness of about 20pm.

3. A television camera tube comprising an evacuated envelope, a transparent, conductive layer on the front wall of said envelope, an electron beam source and a target plate of photo-conductive material on said conductive layer and positioned to be scanned by the electron beam within the envelope, a gauze electrode adjacent and parallel to said target plate, said electrode having a layer of luminescent material on the side of the wires facing said target plate, said luminescent material being energizable by electrons returning from said target plate thereby to produce light which impinges on said target to maintain a predetermined dark current therethrough.

4. A camera tube as claimed in claim 3 in which the photo-conductive material of the target plate consists mainly of a layer of PbO of a thickness of about 20 m. 

1. A television camera tube comprising an evacuated envelope, a transparent conductive layer on the front wall of said envelope, an electron beam source and a target plate of photoconductive material on said conductive layer and positioned to be scanned by the electron beam within the envelope, said electron beam source having electron beam pervious electrode having an aperture therein for the passage of the electron beam and having a planar surface facing the target plate scanned by the electron beam and positioned to receive returning electrons from the target plate, and a luminescent material on said surface of said electrode facing, said target for receiving electrons returning from said target said electrons energizing said luminescent material to produce light which impinges on said target to maintain a predetermined dark current therethrough.
 2. A camera tube as claimed in claim 1, in which the photo-conductive material of the target plate consists mainly of a layer of PbO of a thickness of about 20 Mu m.
 3. A television camera tube comprising an evacuated envelope, a transparent, conductive layer on the front wall of said envelope, an electron beam source and a target plate of photo-conductive material on said conductive layer and positioned to be scanned by the electron beam within the envelope, a gauze electrode adjacent and parallel to said target plate, said electrode having a layer of luminescent material on the side of the wires facing said target plate, said luminescent material being energizable by electrons returning from said target plate thereby to produce light which impinges on said target to maintain a predetermined dark current therethrough.
 4. A camera tube as claimed in claim 3 in which the photo-conductive material of the target plate consists mainly of a layer of PbO of a thickness of about 20 Mu m. 